Alloys  as  Solutions 


BY  - 


John  Alexander  Mathews,  Ph.  D. 


Barnard  Fellow  of  Columbia  University 
and  Carnegie  Research  Scholar  of 
the  Iron  and  Steel  Institute 
of  Great  Britain 


(Reprinted  from  The  Engineering  and  Mining  Journal, 
December  21st  and  28th,  1901) 


ALLOYS  AS  SOLUTIONS 


By  John  Alexander  Mathews 


It  is  truly  remarkable  that  any  class  of  substances 
which  have  been  so  long  known  and  studied  as  the 
alloys  have  been  should  have  hidden  the  secrets 
of  their  inner  molecular  constitution  so  completely 
until  within  a  little  more  than  a  decade.  The  art 
of  metallurgy  is  a  venerable  one,  and  wonder¬ 
ful  achievements  were  accomplished  by  the  earliest 
civilized  peoples  in  practicing  metallurgy  as  an 
empirical  art  and  with  the  crudest  of  appliances. 
The  Chinese  are  supposed  to  have  had  metallic 
currency  for  at  least  4,000  years;  the  Lydians  de¬ 
picted  nymphs,  deities  and  animals  upon  metallic 
coins  in  the  eighth  century  B.  C.,  and  the  coins  of 
Greece  dating  300  B.  C.  show  a  skill  of  workman¬ 
ship  not  excelled  by  modern  engravers.  We  may, 
from  these  evidences  of  the  advances  in  civiliza¬ 
tion,  conclude  that  the  knowledge  of  metals  and 
how  to  obtain  and  utilize  them  was  of  very  early 
origin. 

Historical  Review. — Six  metals  are  mentioned  in 
the  Old  Testament  scriptures,  and  from  this  and 
other  sources  we  know  that  brass,  bronze,  solders 
and  other  alloys  were  all  well  known  before  the 
Christian  era.  We  have  records  of  the  metal¬ 
lurgical  processes  of  cupellation,  amalgamation 
and  smelting  of  an  equally  early  date.  During  all 
the  centuries  from  that  time  to  the  present,  men 
have  been  discovering  new  metails  and  mixing 
them  in  new  proportions  hoping  either  by  design 
or  chance  to  produce  improved  materials  suitable 
co  for  special  purposes.  The  shrewd  observers  of  all 
ages  seem  to  have  been  aware  of  the  profound 
changes  which  the  properties  of  metals  undergo 


4 


ALLOYS  AS  SOLUTIONS. 


when  mixed;  even  though  one  metal  may  be  add¬ 
ed  in  but  minute  quantities  the  result  is  often  prac¬ 
tically  a  new  metal,  and  to  these  early  students 
these  were  considered  in  very  truth  new  metals. 
The  ancients  did  not  recognize  brass  as  an  alloy. 
They  only  knew  that  when  copper  was  melted  with 
a  certain  earth  (zinc  ore)  it  became  more  yellow 
and  golden  in  appearance.  -  This  single  experiment 
no  doubt  had  much  to  do  with  the  development 
of  the  alchemistic  idea  of  the  transmutability  of 
metals.  It  must  be  remembered  that  although  the 
speculative  philosophy  of  the  ancient  Greeks  did 
much  to  hinder  the  development  of  modern  scien¬ 
tific  methods  and  imposed  strong  opposition  to 
physical  research,  yet  it  did  much  to  stimulate 
thought  about  natural  law.  The  teachings  of  Aris¬ 
totle  and  Plato  must  certainly  have  inculcated  a 
deep  feeling  of  respect  for  the  dignity  of  science, 
but  this  spirit  was  held  in  check  for  many  centuries 
by  reason  of  the  evolution  of  the  rigid  dogmatism 
of  the  Christian  era.  Encouraging  signs  of  im¬ 
provement  in  the  way  of  a  revival  of  learning  dur¬ 
ing  the  time  and  through  the  influence  of  Alfred 
the  Great  were  offset  by  the  more  than  equal  re¬ 
vival  in  ecclesiasticism  at  the  same  period. 

The  work  of  two  or  three  men  left  a  lasting  im¬ 
pression  upon  scientific  metallurgy  in  spite  of  all 
opposition.  Geber,  in  the  eighth  century,  and  Agri¬ 
cola  and  Biringuccio,  in  the  fifteenth  century,  re¬ 
corded  many  discoveries  as  the  results  of  their 
studies  and  experiments  upon  the  properties  of 
metals.  These  men,  who  were  in  advance  of  their 
times,  lived  during  the  days  when  the  church  heap¬ 
ed  contempt  upon  all  investigations  of  nature  be¬ 
cause  of  a  belief  in  their  futility.  Men  of  inquiring 
minds  had  to  work  in  secret,  and  thus  the  belief  in 
the  occult,  magic  and  witchcraft  were  mightily 
stimulated.  These  were  the  days  when  papal  bulls 
still  roared  that  the  earth  was  not  round,  because 
in  Holy  Writ  the  “corners  of  the  earth”  are  men¬ 
tioned.  But  even  after  the  domination  of  scholasti- 


ALLOYS  AS  SOLUTIONS. 


5 


cism  had  in  great  part  ceased,  no  one  seems  to  have 
advanced  any  idea  as  to  the  nature  of  metallic  al¬ 
loys.  Oxidation  and  reduction  had  long  been  sub¬ 
jects  for  study,  and  one  of  the  very  earliest  of  the 
Royal  Society’s  investigations  was  into  the  cause 
of  the  gain  in  weight  of  lead  when  heated  in  air, 
and  it  was  discovered  that  it  was  due  to  the  ab¬ 
sorption  of  some  constituent  of  the  atmosphere.  It 
is  not  strange  that  this  problem  attracted  such 
early  attention,  for  the  behavior  of  lead  in  the 
fire  had  been  utilized  in  the  refining  of  gold  and 
silver  for  sixteen  hundred  years  and  cupellation 
for  the  sake  of  quantitatively  determining  the  rich¬ 
ness  of  gold  and  silver  alloys  had  been  used  in 
England  from  the  time  of  Roger,  Bishop  of  Salis¬ 
bury,  in  the  reign  of  Henry  I,  to  whom  Du  Cange, 
the  historian,  gives  the  credit  for  “inventing”  as¬ 
saying.  The  bad  practices  in  which  the  coiners 
of  money  had  engaged  during  the  times  of  William 
the  Conqueror  and  his  son,  Rufus,  were  thus  cor¬ 
rected  by  Henry,  and  of  fifty  coiners  examined  by 
Roger  only  four  escaped  punishment. 

In  the  phlogistic  period,  which  may  be  said  to 
comprise  the  end  of  the  seventeenth  and  most  of 
the  eighteenth  centuries,  metallurgy  made  some 
decided  advances.  Bergman  advanced  an  explana¬ 
tion  of  the  distinctions  between  wrought  iron,  cast 
iron  and  steel  as  early  as  1781,  and  even  stated  the 
percentages  of  carbon  (which  he  called  phlogiston) 
that  each  class  contained.  Still  earlier  than  this, 
Reaumur  (1722)  explained  the  hardening  of  steel  by 
assuming  that  the  metal  was  constituted  of  parti¬ 
cles  which  might  be  likened  to  piles  of  shot.  These 
particles  contained  “sulphur  and  salts,”  which,  by 
reheating,  were  driven  into  the  intersticial  spaces, 
and  by  quenching  the  cooling  was  so  quick  that 
they  could  not  get  back  into  the  molecule.  At 
the  end  of  the  eighteenth  century  the  discussion 
of  free  or  combined  carbon  had  begun;  Clouet  car¬ 
bonized  iron  by  means  of  a  diamond  in  1798;  and 
a  year  later  it  was  stated  that  carbon  and  iron  form 


6 


ALLOYS  AS  SOLUTIONS. 


a  true  chemical  compound.  Notwithstanding  these 
great  advances,  no  very  decided  opinions  upon  the 
constitution  of  intermetallic  mixtures  seem  to  have 
been  recorded.  . 

One  could  scarcely  expect  that  the  hard  prob¬ 
lems  of  molecular  physics,  which  a  study  of  alloys 
involves,  would  be  solved  before  the  announcement 
of  a  molecular  theory  of  the  constitution  of  mat¬ 
ter,  nor  could  it  have  made  any  progress  before 
the  period  of  chemical  reform  begun  by  Lavoisier, 
the  period  of  quantitative  investigation.  But  only 
recently,  even  in  this  period  in  which  we  still  la¬ 
bor,  have  the  relations  of  physical  properties  to 
chemical  constitution  attracted  both  chemists  and 
physicists  to  concerted  activity,  and  in  this  field  of 
investigation  alloys  were  last  to  yield  fruitful  re¬ 
sults.  Physical  and  chemical  methods  of  research, 
which  were  entirely  satisfactory  when  applied  to 
bodies  of  other  kinds,  failed  utterly  to  afford  a  ra¬ 
tional  explanation  of  the  molecular  conditions 
existing  in  alloys.  The  solution  of  these  problems 
required  new  methods  and  new  apparatus.  Fortu¬ 
nately,  these  are  at  hand;  they  have  been  slowly 
evolving  and  the  past  decade  has  been  most  fruit¬ 
ful  of  results.  Problems  of  the  greatest  practical 
importance  and  of  extreme  difficulty  have  been 
solved.  Yet  much  remains  to  be  done,  as  we  have 
but  entered  upon  this  new  field  of  applied  science. 
Let  us  review  the  work  so  recently  accomplished 
by  a  host  of  investigators  which,  I  am  sorry  to  say, 
does  not  include  many  Americans.  We  have  not 
done  our  part  in  this  work,  for,  with  few  excep¬ 
tions,  the  great  leaders  in  it  belong  to  the 
school  of  English  and  French  physicists,  chemists 
and  metallurgists.  A  resume  of  their  achievements 
may  be  of  interest  at  this  time,  and  this  is  given 
in  the  pages  that  follow. 

A  most  potent  factor  in  this  recent  progress  has 
been  the  official  recognition  of  the  necessity  of 
study  along  these  lines.  More  than  ten  years  ago 
the  Institution  of  Mechanical  Engineers  of  Great 


ALLOYS  AS  SOLUTIONS. 


7 


Britain  appointed  an  Alloys  Research  Committee, 
which  receives  financial  assistance  from  the  insti¬ 
tution,  and  five  reports  have  been  published  by  it, 
and  a  sixth  report  is,  I  believe,  about  to  appear. 
It  is  in  connection  with  the  work  of  this  committee 
that  Sir  William  Robert-Austen,  his  pupils  and  the 
co-workers  of  his  staff  have  produced  such  excel¬ 
lent  work,  especially  in  pyrometry  and  in  explain¬ 
ing  the  complicated  changes,  allotropic  and  other¬ 
wise,  which  steel  and  cast  iron  undergo.  The  prac¬ 
tical  operations  of  annealing,  tempering,  harden¬ 
ing,  etc.,  can  now  be  conducted  upon  a  scientific 
and  rational  basis.  The  effects  of  thermal  opera¬ 
tions  can  be  predicted  accurately  and  desired  re¬ 
sults  obtained  unfailingly.  It  would  be  too  much 
to  say  that  all  this  is  due  to  this  committee  alone; 
but  if  we  include  also  the  committee  of  the  British 
Association,  appointed  to  investigate  the  nature  of 
alloys,  of  which  Mr.  Neville  is  chairman  and  the 
“Commission  des  Alliages,,,  of  the  Societe  d’En- 
couragement  lTndustrie  Nationale,  as  also  sharing 
the  honors  of  recent  great  advances  in  both  our 
theoretical  and  practical  knowledge  of  alloys,  then 
we  have  recognized  the  three  most  prolific  contribu¬ 
tors  to  our  knowledge  of  metallic  mixtures.  While 
giving  these  official  bodies  full  credit  for  their  dis¬ 
coveries,  we  would  not  detract  in  any  way  from 
the  honor  due  to  many  individual  workers  not  con¬ 
nected  with  committees  who  are  scattered  through 
many  lands.  It  is  safe  to  say,  however,  that  the 
larger  results  of  the  concerted  work  of  commit¬ 
tees  has  had  a  stimulating  effect  upon  the  private 
worker.  One  of  the  greatest  evidences  of  apprecia¬ 
tion  of  the  necessity  for  research  in  connection  with 
alloys  is  that  nearly  every  railroad  in  France  and 
many  metallurgical  concerns  contributed  to  sus¬ 
tain  the  work  of  the  “Commission  des  Alliages.” 

The  purely  scientific  reasons  which  have  made 
possible  the  recent  advances  in  alloys  research 
seem  to  me  to  be  :  (i)  An  increased  knowledge  of 
solutions.  For  this  we  are  undoubtedly  indebted 


8 


ALLOYS  AS  SOLUTIONS. 


in  very  great  measure  to  the  great  Dutch  and 
German  physical  chemists — Vant  Hoff,  Rooze- 
boom,  Ostwald,  Nernst  and  others.  (2)  The 
development  of  metallography.  This  seems  to  have 
originated  with  Dr.  Sorby,  of  Sheffield.  Although 
he  wrote  about  the  microstructure  of  meteoric  iron 
as  early  as  1864,  the  progress  of  metallography  was 
very  slow.  Only  within  the  past  ten  or  fifteen  years 
have  even  metallurgists  begun  to  recognize  what  a 
valuable  aid  to  research  it  is  and  unfortunately  the 
manufacturers  have  not  yet  given  it  just  recogni¬ 
tion.  Recent  progress  in  this  branch  of  applied 
sciences  is  most  closely  attached  to  the  names  of 
Stead,  Martens,  Osmond,  Le  Chatelier,  Charpy,  Rob- 
erts-Austen,  Howe  and  Sauveur.  (3)  Improvements 
in  pyrometry.  The  study  of  metals,  alloys,  furnace  re¬ 
actions,  etc.,  was  retarded  for  many  years  because  of 
the  lack  of  the  suitable  means  of  measuring  accurately 
very  high  temperatures.  Such  means  as  have  been 
available  for  these  purposes  in  past  years  were  not 
suitable  for  general  use  around  shops  and  metal¬ 
lurgical  works  and  were  inconvenient  even  for  the 
laboratory.  Now,  however,  there  are  at  least  two 
reliable  pyrometers  which  are  conveniently  mani¬ 
pulated  and  lend  themselves  readily  to  accurate 
measurements  of  high  temperatures.  The  Siemens 
electrical  resistance  pyrometer,  as  perfected  by  Cal- 
lendar  and  Griffiths,  gives  wonderfully  accurate 
readings  within  certain  limits.  For  use  both  in  the 
laboratory  and  in  the  factory  it  does  not  seem  as 
popular — in  the  United  States  at  least — as  the  Le 
Chatelier  thermo-electric  couple.  The  very  accu¬ 
rate  work  of  Messrs.  Heycock  and  Neville  is  per¬ 
haps  as  good  a  recommendation  for  the  possibilities 
of  the  electrical  resistance  pyrometer  as  can  be 
cited.  The  particular  advantages  of  the  thermo¬ 
electric  couple  as  applied  to  alloys  research  are — 
its  accuracy  at  high  temperatures,  its  ability  to  be 
used  with  very  small  masses  of  heated  substances, 
and,  indirectly,  its  ability  to  be  used  in  conjunc¬ 
tion  with  a  photo-autographic  recorder,  as  has  been 


ALLOYS  AS  SOLUTIONS.  Q 

so  well  demonstrated  by  Sir  William  Roberts- 
Austen. 

Crystalline  Growth  in  Metals. — Before  we  enter 
upon  the  consideration  of  alloys,  it.  may  be  well  to 
note  a  few  general  properties  of  metals,  particular¬ 
ly  the  mode  of  crystalline  growth  and  the  effects  of 
strain.  To  both  of  these  subjects  Professor  Ewing, 
of  Cambridge,  has  given  much  attention  and  has 
explained  the  phenomena  of  these  operations  very 
clearly.  He  states  that  all  solid  metals  are  crystal¬ 
line,  even  though  when  examined  by  the  micro¬ 
scope  the  individual  crystals  or  crystalline  grains 
may  not  present  a  simple  geometrical  outline.  The 
essential  point  is  that  the  particles  composing  the 
mass  of  a  crystal  lie  in  one  direction,  i.e.,  have  the 
same  plane  of  orientation.  Mr.  G.  T.  Beilby  read 
a  paper  at  the  meeting  of  the  British  Association 
held  in  Glasgow,  September,  1901,  upon  “The  Minute 
Structure  of  Metals,”  and  cites  evidence  to  show 
that  he  has  identified  minute  spicules  or  scales 
showing  remarkable  uniformity  of  size  and  appear¬ 
ance  in  metals  of  all  the  leading  groups.  These 
under  mechanical  treatment  may  be  converted  into 
transparent  glass-like  substance  and  may  again  re¬ 
appear  as  spicules  unchanged  in  size.  The  diam¬ 
eter  of  the  scales  is  estimated  to  range  from  1-300 
to  1-400  of  a  millimeter.  The  metals  examined  by 
Mr.  Beilby  included  gold,  silver,  platinum,  cobalt, 
nickel,  chromium,  iron,  copper,  lead,  bismuth,  anti¬ 
mony,  tin,  cadmium,  magnesium,  aluminum,  zinc 
and  sodium.  Brasses  and  bronzes  were  also  ex¬ 
amined.  The  author  says  that  the  crystalline  faces 
and  cleavages  of  such  metals  as  antimony,  bismuth 
and  zinc  exhibit  the  same  features  as  the  softer 
and  more  malleable  metals,  being  covered  with  a 
film  of  transparent  metal,  while  in  fractures  at  right 
angles  to  cleavage  planes,  scales  are  distinctly  seen. 
Mr.  Beilby’s  conclusions  are :  “The  persistence  of 
these  minute  scales  or  spicules  under  all  kinds  of 
mechanical  and  thermal  treatment,  the  remarkable 
uniformity  of  their  size  and  appearance  in  metals 


10 


ALLOYS  AS  SOLUTIONS. 


of  all  the  leading  groups,  their  disappearance  into 
the  transparent  form  and  their  reappearance  again, 
apparently  unchanged  in  size  or  otherwise,  seem  to 
afford  fair  ground  for  the  conjecture  that  they 
are  in  some  way  definite  units  in  the  structure  of 
metals.” 

It  may  be  that  these  are  the  “units”  whose  ar¬ 
rangement  in  certain  definite  orders  constitute 
crystals,  and  this  similarity  of  disposition  of  par¬ 
ticles  is  what  constitutes  orientation.  Professor 
Ewing  explains  crystallization  in  metals  in  this  way: 
the  formation  of  crystals  must  be  assumed  to  start 
simultaneously  at  many  points.  The  crystals  grow 
until  they  touch  one  another,  and,  although  indi¬ 
vidual  crystals  are  similar,  it  is  not  necessary  that 
the  corresponding  axes  of  any  two  of  them  lie  in 
the  same  plane.  No  doubt  each  crystal  is  of  per¬ 
fect  form  so  long  as  it  is  free  to  grow  in  all  direc¬ 
tions;  when  the  crystals  touch,  their  symmetrical 
growth  is  hindered,  but  the  orientation  of  the  sub¬ 
sequent  particles  which  attach  themselves  is  not 
changed.  It  is  for  this  reason  that  orientation 
rather  than  geometrical  outline  is  considered  the 
essential  feature  of  crystalline  growth.  In  the  case 
of  commercial  metals,  more  or  less  impure,  the  im¬ 
purities  are  cast  out  by  the  growing  crystals,  and 
being  in  fact  alloys  of  the  admixed  metals  with  the 
still  fluid  principal  metal,  they  have  a  lower  melt¬ 
ing  point  than  the  pure  metal.  Therefore,  they 
solidify  last  and  form  an  investing  cement  which 
holds  together  the  primary  crystals. 

The  Effect  of  Strain. — When  a  polished  metal  is 
subjected  to  strain  there  is  no  change  noticeable 
until  the  stress  exceeds  the  elastic  limit.  When, 
however,  the  plastic  stage  is  reached,  there  will 
appear  dark  lines  more  or  less  perpendicular  to  the 
direction  of  stress.  This,  Professor  Ewing  says, 
is  not  due  to  Assuring,  for  by  changing  the  direc¬ 
tion  of  the  rays  which  illuminate  the  specimen 
under  observation  for  vertical  to  oblique,  it  will 
be  seen  that  the  erstwhile  dark  lines  appear  light, 


ALLOYS  AS  SOLUTIONS. 


II 


while  the  background  has  changed  from  light  to 
dark.  Fissures  would  not  have  reflected  light  in 
any  case;  the  permanent  elongation  is  due  to  slip¬ 
ping  of  the  components  of  the  crystals  past  each 
other.  That  is,  the  surface  which  was  originally 
plain,  presents,  after  stress  has  been  applied  beyond 
the  elastic  limit,  the  appearance  of  minute  steps. 
It  is  as  though  one  should  take  a  pack  of  playing 
cards  with  their  ends  presenting  a  smooth  surface. 
Now  if  the  thickness  of  the  pack  of  cards  were  one 
inch,  and  the  pack  were  bent,  the  end  surface  of 
the  pack  would  measure  more  than  an  inch  and 
would  consist  of  little  steps,  not  only  the  ends,  but 
also  a  part  of  the  flat  surfaces  of  the  cards  being  ex¬ 
posed. 

In  the  case  of  metals,  the  slip-lines  need  not 
necessarily  lie  in  one  plane.  As  many  as  four  sets 
of  parallel  slip  lines  have  been  noticed  by  Professor 
Ewing.  Mr.  William  Campbell  worked  upon  this 
phenomenon  independently  at  about  the  time  that 
Professor  Ewing  was  making  these  interesting  ob¬ 
servations.  He  experimented  with  tin,  while  Pro¬ 
fessor  Ewing  worked  chiefly  with  lead,  and  it  is 
interesting  to  note  how  completely  their  results 
agree. 

The  growth  of  crystals  from  a  solidifying  molten 
mass  is  easy  to  comprehend,  but  why  is  it  that 
crystals  grow  in  a  solid  mass  of  metal  not  only  by 
annealing  but  even  at  ordinary  temperatures?  Big 
crystals  actually  consume  the  little  ones.  A  bit  of 
rolled  tin  which  was  allowed  to  stand  for  eighteen 
months  at  ordinary  temperatures  in  the  laboratory 
showed  a  marked  change  in  appearance,  due  to 
growth  of  its  crystals.  If  a  specimen  of  tin  is  put 
upon  a  hot  plate  and  heated  to,  say,  200°  C.  for 
ten  days,  a  growth  such  as  is  shown  in  Fig.  1  is 
obtained,  which  shows  the  tin  before  and  after 
annealing.  This  photograph  is  one  of  Mr. 
Campbell’s,  and  is  a  little  under  actual  size.  It 
also  shows  very  well  the  different  orientation  of 
the  crystals.  In  fact,  the  growth  of  these  crystals 


12 


ALLOYS  AS  SOLUTIONS. 


was  so  rapid  that  a  microsection  through  them 
reveals  slip-lines,  probably  due  to  this  rapid  growth. 
The  fact  that  the  slip  lines  are  only  parallel  within 
the  same  crystal  seems  to  confirm  this  view,  for, 
had  the  stress  been  applied  longitudinally  by  artifi¬ 
cial  means  all  the  slip-lines  would  have  been  more 
or  less  normal  to  the  direction  of  the  stress. 

With  regard  to  the  growth  of  crystals  in  strained 
metals  when  they  are  subsequently  warmed,  Mr. 
Rosenhain  has  offered  this  explanation:  In  strained 
metals  the  layer  of  enveloping  alloy  surrounding 
the  separate  crystals  becomes  broken,  and  the  dif- 


FIG.  i. 


ference  of  potential  between  the  metal  and  the  al¬ 
loy  set  up  by  electrolytic  action  causes  a  solution 
of  the  crystal  on  one  side  of  the  alloy  and  a  de¬ 
posit  on  the  adjoining  side.  This  he  verified  by  the 
following  experiment:  Two  pieces  of  lead  were 
scraped  clean  and  welded  by  the  application  of  great 
pressure.  A  micro-section  through  the  weld  show¬ 
ed  that  the  crystals  in  neither  piece  extended  ac- 
cross  the  weld.  When,  however,  between  two  such 
surfaces  a  little  powdered  tin  was  sprinkled  and 
pressure  applied,  a  micro-section  revealed  that 
electrolytic  action  had  taken  place  and  that  crystals 
from  either  side  extended  across  the  weld.  In¬ 
genious  as  is  this  explanation,  yet  I  am  not  certain 
that  the  proof  is  conclusive.  If  I  mistake  not,  at 


ALLOYS  AS  SOLUTIONS. 


13 


the  last  Cantor  lectures  before  the  Society  of  Arts,1 
Roberts-Austen  and  Rose  showed  crystals  of 
gold  extending  across  a  weld,  although  the  purest 
standard  gold  was  used  in  the  experiment.  In  fact, 
long  annealing  of  this  gold  obliterated  the  weld  al¬ 
most  completely. 

Freezing  Point  of  Metals. — With  water  and  many 
other  liquids  we  know  that  it  is  quite  possible  to 
lower  the  temperature  to  a  considerable  amount 
below  their  freezing-point  without  the  separation 
of  any  solid.  On  the  other  hand,  it  is  not  known 
that  any  solid  may  be  heated  above  its  melting- 
point  without  becoming  liquid.  The  property  first 
mentioned  is  known  as  surfusion.  On  account  of 
this  phenomenon  it  would  be  theoretically  prefer¬ 
able  to  determine  the  melting  rather  than  the  freez¬ 
ing  point  of  metals,  but  practically  this  is  a  very 
difficult  task,  while  freezing-points  are  in  most 
cases  easily  determined  and  surfusion  need  not  in¬ 
terfere  with  their  accuracy  in  the  least,  for  when 
solid  matter  begins  to  separate  from  a  pure  liquid 
thus  cooled,  the  temperature  rises  rapidly  to  the 
true  freezing  point  and  remains  constant  until  the 
whole  mass  is  solid.  Sir  William  Roberts-Austen 
cites  an  instance  in  which  tin  was  cooled  20°  C.  be¬ 
low  its  freezing  point  without  solidification,  and  I 
have  frequently  observed  the  same  result  in  tin  and 
some  other  metals,  but  to  a  less  degree.  In  work¬ 
ing  with  metals  the  first  point  at  which  solids  sepa¬ 
rate  is  taken  as  the  freezing  point,  and  surfusion 
need  not  obscure  this  point,  particularly  if  an 
autographic  record  of  the  cooling  curve  is  taken. 
If  we  plot  a  typical  cooling  curve  of  a  pure  metal, 
using  temperature  and  time  as  the  co-ordinates, 
we  obtain  a  curve  represented  by  A  B,  Fig.  2,  hav¬ 
ing  two  very  distinct  angles,  while  a  portion  is 
horizontal,  i.  e.,  the  temperature  remains  constant 
during  the  whole  period  of  solidification.  If  sur¬ 
fusion  takes  place,  and  it  is  more  common  in  pure 
metals  than  in  alloys,  the  curve  A'  B'  represents 


0)  J.  Soc.  of  Arts.  (1901)  p.  851. 


14  ALLOYS  AS  SOLUTIONS. 

what  happens.  The  temperature  falls  below  the 
real  freezing  point,  and  then  rises  abruptly  to  that 
point  and  remains  constant  as  in  A  B  until  solidifi¬ 
cation  is  complete.  In  impure  metals — and  since 
the  impurities  are  usually  metallic,  they  are  really 
alloys — the  temperature  does  not  remain  constant 
at  all,  but  a  decided  change  in  direction  is  noticed 
as  shown  in  A"  B".  The  greater  the  amount  of 
impurity,  the  more  rounded  will  appear  the  cool¬ 


ing  curve.  Since  these  are  really  the  curves  of 
alloys  and  solid  solutions,  they  will  be  explained 
at  greater  length  subsequently.  This  third  type, 
A"  B"  is  introduced  here  because  it  shows  that 
the  nature  of  the  cooling  curve  gives  us  a  clue  as 
to  the  purity  of  the  metal  under  examination.  To 
explain  surfusion,  we  need  only  remember  that 
fusion  is  always  attended  by  absorption  of  heat  and 
solidification  by  evolution  of  heat.  When  a  :<sur- 


ALLOYS  AS  SOLUTIONS. 


15 


fused”  substance  begins  to  solidify  it  disengages 
heat  and  the  temperature  rises  until  the  melting 
point  is  reached.  No  further  rise  takes  place  be¬ 
cause  at  this  temperature  only,  liquid  and  solid 
are  in  equilibrium.  Any  further  change  of  liquid 
to  solid  or  sodid  to  liquid  must  be  effected  by  ab¬ 
stracting  or  adding  heat.  In  practice,  either  heat 
is  lost  by  radiation  and  the  whole  mass  solidifies, 
or  is  supplied  by  artificial  warming,  and  the  whole 
mass  becomes  liquid. 

Binary  Alloys. — Having  now  noted  some  of  the 
properties  of  pure  metals  we  may  next  consider 
what  happens  when  two  metals  are  melted  together. 
We  shall  not  at  this  time  consider  ternary  or  more 
complex  alloys.  The  first  thing  noticed  upon  melt¬ 
ing  together  two  metals  is  that  (1)  they  mix  in  all 
proportions,  or  (2)  they  do  not.  In  the  first  in¬ 
stance,  under  proper  conditions  an  approximately 
homogeneous  solid  mass  is  likely  to  result  when  the 
molten  alloy  is  cooled.  In  the  second  case,  on  cool¬ 
ing  it  will  be  found  that  the  metals  have  separated 
into  layers,  the  lighter  metal  on  top.  Each  layer, 
however,  will  be  found  upon  analysis  to  contain  a 
certain  amount  of  the  other  metal  in  solution,  just 
as  ether  and  water  dissolve  small  quantities  each 
of  the  other.  As  between  certain  metals  and  solid 
non-metallic  elements  it  seems  as  though  they  were 
absolutely  insoluble  in  one  another  at  ordinary 
temperatures,  although  they  may  be  quite  soluble 
at  high  temperatures.  For  example,  M.  Moissan 
tells  us  that  metals  of  the  platinum  group  readily 
dissolve  several  per  cent  of  carbon  at  the  temper¬ 
ature  of  the  electric  furnace,  but  cast  it  all  out  as 
graphite  on  cooling.  It  has  not  yet  been  deter¬ 
mined  whether  the  graphite,  which  separates  in 
cast  iron,  contains  iron  or  not.  It  seems  to  me 
that  we  shall  come  nearer  the  truth  if  we  consider 
such  cases  as  solutions  of  almost  infinite  dilution, 
rather  than  as  absolutely  pure  metal  or  non-metal. 

We  have  just  spoken  of  metals  dissolving  metals, 
and  it  is  this  conception  of  alloys  as  solutions — 


l6  ALLOYS  AS  SOLUTIONS. 

solid  solutions — that  has  been  steadily  progressing 
in  favor  in  recent  years  and  which  we  hope  both 
to  explain  and  to  confirm  by  citing  the  results  of 
the  experiments  of  many  investigators.  I  remember 
hearing  a  teacher,  after  explaining  the  nature  of  the 
elements  and  their  tendency  to  combine  in  definite 
propositions,  assure  the  pupils  that  “alloys  are  al¬ 
loys’’ — a  delightfully  simple  statement,  but  future 
pupils  as  well  as  their  teachers  will  be  expected  to 
know  a  little  more  about  them. 

Very  striking  analogies  exist  between  ordinary 
solutions  and  alloys;  in  fact,  to  point  out  these 
similarities  is  to  give  our  present  conception  of 
the  constitution  of  alloys.  ( Van’t .) 

Solid  Solutions. — This  expression  is  due  to  Vant 
Hoff,  but  Roberts-Austen  has  done  most  to  make 
it  familiar  to  metallurgists.  Our  text-books  upon 
physical  chemistry  contain  much  information  upon 
the  subjects  of  solutions  of  gases  in  liquids,  li¬ 
quids  in  liquids,  and  solids  in  liquids,  but  are  for 
the  most  part  silent  upon  the  subject  of  the  solu¬ 
tions  of  solids  in  solids.  The  extension  of  our 
knowledge  upon  this  subject  is  very  recent  and 
mostly  confined  to  technical  periodical  literature  of 
recent  dates.  Professor  Roozeboom  and  his  pu¬ 
pils  have  carried  on  researches  upon  the  nature  of 
fused  mixtures  of  salts  and  their  behavior  upon 
cooling.  This  work  throws  considerable  light  up¬ 
on  the  problems  of  chemical  equilibrium  in  alloys. 
As  defined  by  Sir  William  Roberts-Austen  a  Solid 
solution  is  “a  homogeneous  mixture  of  two  or 
more  substances  in  the  solid  state.”  In  metals 
no  one  has  worked  as  yet  with  non-crystalline  mix¬ 
tures,  and  solid  solutions  of  metals  when  crystal¬ 
line  are  solid  “isomorphous  mixtures,”  or  “mixed 
crystals.”  The  use  of  these  three  terms  to  denote 
one  condition  is  confusing  and  unnecessary,  and  as 
objections  exist  to  the  last  two  synonyms,  their 
use  should  be  discontinued.  The  term  “mixed  crys¬ 
tals”  is  particularly  apt  to  convey  a  wrong  im¬ 
pression  to  one  who  sees  this  name  for  the  first 


ALLOYS  AS  SOLUTIONS.  1 7 

time,  especially  if  it  is  unaccompanied  by  a  defini¬ 
tion  or  explanation. 

Mr.  Stead,  in  the  Journal  of  the  Iron  and  Steel 
Institute ,  1900,  classifies  solid  solutions  as  follows: 

(1)  Those  in  which  one  constituent  of  an  alloy 
in  crystallizing  retains  a  portion  of  the  other  homo¬ 
geneously  diffused  through  its  whole  crystalline 
mass. 

(2)  Those  in  which  during  crystallization  the  cen¬ 
tral  portion  of  the  crystals  contains  less  of  the  dis¬ 
solved  substance  than  their  external  boundaries. 

(3)  Those  in  which  the  metals  form  a  definite 
compound,  which  is  retained  in  solid  solution  in 
the  excess  of  metal  or  metals.  I  should  like  to 
add  that  the  “compound”  may  be  the  “solvent,”  in 
the  sense  in  which  those  terms  are  here  used. 

The  condition  described  in  (2)  is  that  of  an  unsta¬ 
ble  system.  Crystallization  has  taken  place  more  rap¬ 
idly  than  diffusion,  and  in  such  cases  where  crystals 
are  not  of  uniform  composition  from  center  to  out¬ 
side  complete  homogeneity  could  probably  be  ob¬ 
tained  by  annealing.  Mr.  Stead  adds  a  fourth  class. 

(4)  Those  in  which  the  non-metallic  elements 
form  definite  compounds  with  a  portion  of  the  dis¬ 
solving  metal  that  remain  in  solid  solution.  This 
class  would  include  the  property  of  metals,  such 
as  iron  and  copper,  to  dissolve  a  portion  of  their 
oxides.  Such  solutions  have  been  little  studied. 

In  the  case  of  metals  which  do  not  mix  in  all 
proportions,  the  two  layers  formed  on  cooling  con¬ 
sist  of  solid  solutions  belonging  to  one  of  the 
types  just  mentioned.  Some  of  the  pairs  of  metals 
which  do  not  mix  in  all  proportions  at  ordinary 
temperatures  are  Zn-Pb,  Zn-Bi,  Pb-Al,  Bi-Al,  Cd-Al. 
These  have  all  been  studied  by  Dr.  Alden  Wright,* 
and  Mr.  Campbell  and  myself  have  repeated  and 
verified  a  part  of  Dr.  Wright's  work  upon  the 
aluminum  alloys.  In  all  these  pairs  of  metals  the 
relative  solubility  is  a  function  of  the  temperature; 


*(Proc.  Roy.  Soc.  1898-1893) 


ALLOYS  AS  SOLUTIONS. 

therefore,  it  appears  likely  that  at  some  temper¬ 
ature  solubility  is  complete.  Accordingly  we  may 
represent  the  condition  existing  between  these  and 
any  other  similar  pairs  of  metals  by  a  “critical 
curve  (Fig.  3).  A  and  B  are  the  metals  which  are 
not  entirely  miscible  at  ordinary  temperatures,  but 
which  form  layers.  The  metal  A  contains  the  quan¬ 
tity  of  B  represented  by  the  space  A  x  on  the  com¬ 
position  line.  Similarly  the  metal  B  contains  B  y 
per  cent  of  A.  With  increasing  temperature  the 
mutual  solubilities  increase,  and  at  a  temperature 
z  there  is  but  one  solution.  Alloys  represented 
by  percentage  compositions,  which  fall  outside  the 


A  Composition  £ 


curve  x  z  y  are  homogeneous.  Those  whose  com¬ 
position  is  represented  by  percentages  within  x  z  y 
may  be  considered  as  similar  to  emulsions  at  all 
temperatures  below  the  curve  at  which  they  are 
liquid.  They  are  incapable  of  remaining  homo¬ 
geneous.  To  such  alloys,  Sir  George  Gabriel 
Stokes  gives  the  name  of  ideal  (i.e.,  unreal,  imagi¬ 
nary)  alloys  while  those  falling  outside  the  area 
x  z  y  he  calls  real  alloys.  The  distinction  between 
such  pairs  of  metals  as  form  separate  layers  on 
cooling,  and  the  more  or  less  laminated  or  striated 
structure  produced  upon  solidification  of  eutectic 
alloys,  will  be  pointed  out  later. 


ALLOYS  AS  SOLUTIONS.  19 

Molecular  Freedom  in  Solids. — It  must  not  be 
supposed  that  because  certain  bodies  are  desig¬ 
nated  “solid”  that  entire  absence  of  molecular  mo¬ 
bility  is  implied.  Such  an  idea  is  far  from  the  truth; 
the  difference  in  mobility  of  the  molecules  in  “sol¬ 
ids”  and  “liquids”  is  one  of  degree  only.  On  the 
boundary  between  solids  and  liquids  substances  are 
said  to  be  viscous  or  plastic.  Just  as  crystals  of  a 
soluble  salt  placed  in  contact  with  water  tend  to 
become  dissolved  and  uniformly  distributed 
throughout  the  solvent;  so  metals,  even  at  nor¬ 
mal  temperatures,  tend  to  form  uniform  solutions; 
this  tendency,  though  slight,  is  none  the  less  real. 
Metals  even  tend  to  evaporate  and  to  be  surround¬ 
ed  by  their  own  vapor.  Robert  Boyle  thought 
that  “even  gold”  had  its  “little  atmosphere.”  From 
analogy  with  salt  solutions  we  may  suppose 
that  one  metal  dissolved  in  another  shows  a  small 
expansive  tendency  to  escape  from  solution;  in 
other  words,  exhibits  osmotic  pressure. 

Professor  W.  Spring,  of  Liege,  demonstrated  the 
fluidity  of  solids,  if  I  may  use  that  expression,  in 
his  famous  experiments  upon  the  behavior  of  metals 
under  exceedingly  high  pressures — ten  thousand  at¬ 
mospheres,  more  or  less.  He  showed  that  by  pres¬ 
sure  alone : 

(1)  Metals  can  be  made  to  flow.  The  familiar 
operation  of  striking  coins  illustrates  this  property 
of  metals  and  alloys.  A  medal  has  recently  been 
struck  in  steel. 

(2)  Solids,  both  metals  and  salts,  like  liquids  and 
gases,  possess  perfect  elasticity  and  suffer  no  per¬ 
manent  diminution  in  yolume  by  pressure.  Strain, 
torsion  or  bending  may  produce  permanent  de¬ 
formation  in  a  metal  or  alloy,  but  there  is  no  limit 
of  elasticity  in  regard  to  diminution  in  volume, 
for  when  the  pressure  is  removed  the  solid  as¬ 
sumes  its  original  volume.  An  exception  to  this 
is  cited  by  Professor  Spring  in  his  next  generaliza¬ 
tion,  viz.: 

(3)  Allotropic  changes  may  result  from  pressure. 


20  ALLOYS  AS  SOLUTIONS. 

Prismatic  sulphur  becomes  octahedral  sulphur  and 
amorphous  arsenic  becomes  crystalline.  In  both 
these  cases  the  second  allotropic  form  is  denser 
than  the  first,  and  from  these  and  other  experi¬ 
ments  Professor  Spring  argues  that  allotropic 
tranisformaitio,ns  by  pressure  show  that  fmatter 
takes  the  state  which  corresponds  to  the  volume 
it  is  obliged  to  occupy.  Moissan’s  brilliant  experi¬ 
ments  upon  the  production  of  artificial  diamonds  il¬ 
lustrates  this  principle. 

(4)  Metallic  filings  are  converted  by  pressure 
alone  into  solid  masses,  just  as  if  they  had  been 
fused.  Similarly  mixed  filings  yield  alloys;  e.g., 
brass  was  produced  by  compressing  clean,  flat  sur¬ 
faces  of  zinc  and  copper.  Reactions  of  double 
decomposition  between  dry  salts  and  the  produc¬ 
tion  of  the  highly  colored  regulus  of  Venus  (Cu2 
Sb)  show  that  not  only  intimate  agglutination,  but 
also  chemical  combination  may  take  place  between 
bodies  in  the  solid  state.  I  have  confirmed  some 
of  Professor  Spring’s  results,  but  was  unable  to 
produce  several  typical  intermetallic  compounds 
as  Au  Al2,  Sb  Sn,  Sn  Cu4,  etc. 

Professor  Roberts-Austen  has  demonstrated  the 
fact  of  molecular  mobility  among  solids  in  a  pro¬ 
longed  series  of  very  beautiful  experiments  upon 
the  diffusion  of  gold  into  lead  at  temperatures 
from  250°  C.  down  to  the  ordinary  temperature. 
Graham  showed  long  ago  that  gold  diffusing  into 
molten  lead,  tin,  etc.,  obeyed  Fick’s  law, 
d  v  d2  v 

d't  ~~  ^  d  x2 

where  x  is  the  distance  through  which  dif¬ 
fusion  takes  place  (against  gravity),  v  is  the 
concentration  of  the  diffusing  metal,  t  =  time,  and 
k  is  the  diffusion  constant,  i.e.,  the  quantity  of 
metal  in  grams  diffusing  in  unit  of  area  (1  cm2)  in 
unit  of  time  (1  day)  when  unit  difference  of  con¬ 
centration  (in  grams  per  cm3)  is  maintained  be¬ 
tween  the  sides  of  a  layer  1  cm.  thick.*  That  is, 

*See  Roberts-Austen,  Graham  Lecture  before  the  Philosophi¬ 
cal  Society  of  Glasgow,  1901. 


ALLOYS  AS  SOLUTIONS. 


21 


metals  dissolve  in  metals,  just  as  salts  dissolve  in 
water.  Were  it  not  for  the  operation  of  this 
law  it  would  not  be  such  an  easy  matter  to  pre¬ 
pare  homogeneous  alloys.  For  instance,  with 
relatively  little  stirring  and  relying  upon  diffusion, 
it  is  very  easy,  according  to  Sir  William  Roberts- 
Austen  to  make  1,200  ounces  of  coin  gold  alloy, 
the  first  and  last  pourings  of  which  shall  not  differ 
by  more  than  1-10,000  part  in  fineness.  The  idea 
that  interchange  of  matter  can  take  place  between 
solids  is  not  a  new  one,  as  is  shown  by  the  cemen¬ 
tation  process  of  the  ancient  Hebrews  by  which 
gold  was  purified. 

The  experiments  upon  the  diffusion  of  gold  into 
solid  lead,  which  Roberts-Austen  has  recently  re¬ 
ported  to  the  Royal  Society,  may  be  summarized 
as  follows: 

Gold  placed  in  the  bottom  of  a  tube  filled  with 
lead  and  maintained  at  250°  C.  (790  below 
the  melting  point  of  lead)  appeared  at  the  top  in 
notable  quantities  in  a  month.  At  ioo°  C.  the  rate 
of  diffusion  was  1-100,000  of  that  in  fluid  lead,  and 
in  solid  lead  at  the  ordinary  temperature,  allowed 
to  stand  in  contact  for  four  years,  unmistakable 
signs  of  diffusion  took  place,  but  its  rate  was  such 
that  in  one  thousand  years  the  diffusion  would 
equal  that  taking  place  in  one  day  in  lead  just 
molten. 

The  figure  4  shows  another  sort  of  change  which 
may  take  place  in  solid  materials.  An  alloy  con¬ 
taining  88.7  per  cent  Cu  and  11.3  per  cent  Al. 
corresponds  closely  to  the  compound  Cu3  Al.  It 
should,  therefore,  appear  almost  homogeneous,  and 
so  it  does  when  quickly  cooled.  When  slowly  cool¬ 
ed,  however,  it  changes  at  500°  C.,  or  nearly 
6oo°  below  its  freezing  point,  and  presents  the 
variegated  appearance  shown  in  Fig.  4,  which  no 
metallographist  would  care  to  call  a  typical  struc¬ 
ture  for  an  intermetallic  compound. 

We  apply  practically  the  phenomenon  of  molecu¬ 
lar  mobility  in  solids  in  all  annealing  operations. 


22 


ALLOYS  AS  SOLUTIONS. 


By  this  means  the  ill  effects  of  strain  due  to  sud¬ 
den  cooling  are  remedied,  and  equilibrium  is  estab¬ 
lished  in  articles  of  metal,  glass,  enamel  and  other 
materials.  The  rate  of  diffusion  of  gold  in  lead 
at  the  various  temperatures  is  an  indication  of  the 
rapid  increase  in  molecular  mobility  which  results 
by  very  moderate  increments  in  temperature.  This 
knowledge  is  applied  whenever  we  resort  to  moder¬ 
ately  elevated  temperatures  in  all  sorts  of  anneal¬ 
ing  operations.  Steel  is  very  sensibly  annealed  at 


fig.  4. 

temperatures  as  much  as  i,ooo°  C.  below  its  melt¬ 
ing  point. 

Behavior  of  Binary  Alloys  During  Cooling  from 
Fused  State. — It  has  been  stated  that  metals  may 
or  may  not  mix  in  all  proportions.  If  they  do  mix, 
we  may  further  distinguish  those  cases  in  which  (1) 
chemical  combination  does  not  take  place,  and  (2) 
those  in  which  chemical  combination  does  take 
place,  and  one  or  more  intermetallic  compounds 
result.  The  reality  of  the  existence  of  intermetal¬ 
lic  compounds  is  undoubted,  yet,  so  far  as  I  know, 
no  mention  is  made  of  them  in  the  standard  books 
on  general  chemistry.  Indeed  very  little  is  known 
in  regard  to  the  nature  of  the  affinity  between 
metals.  It  does  not  seem  necessary  to  suppose 
that  the  laws  applying  to  combinations  between 
elements  or  radicals  which  are  relatively  electro¬ 
positive  and  electro-negative  be  equally  applica- 


ALLOYS  AS  SOLUTIONS. 


23 


ble  here;  and  our  conceptions  of  valency  are  seri¬ 
ously  shattered  if  we  attempt  to  reconcile  them 
to  intermetallic  compounds.  The  difficulties  in 
studying  intermetallic  compounds  are  very  great, 
because  they  are  troublesome  to  isolate,  and  in 
many  of  their  properties  they  resemble  the  metals. 
Frequently,  too,  they  are  quite  unstable  and  show 
marked  dissociation.  Chemical  methods  of  study¬ 
ing  them  are  of  very  limited  application;  filtering 
off  solidified  intermetallic  compounds  from  a  still 
fluid  metallic  mother  liquor  at  high  temperatures 
presents  extreme  difficulties  and  volatilization  of 
the  excess  of  solvent  metal  is  rarely  applicable. 
In  a  few  cases  the  heat  of  formation  can  be  de¬ 
termined  and  electrical  methods  by  which  the  po¬ 
tential  of  binary  alloys  is  compared  with  that  of 
the  more  positive  metal  in  the  alloy  may  yield  im¬ 
portant  evidence  as  to  the  solubility  of  one  metal 
in  another  in  the  solid  state.  The  microscopic  evi¬ 
dence  and  that  of  the  freezing  point  curves  require 
some  amplification,  for  they  are  not  wholly  satis¬ 
factory  in  this  regard.  (See  Herschkowitz,,  Zeits. 
Phys.  Chemie,  XXVII.,  p.  113,  and  Laurie,  Trans. 
Chem.  Soc.  (1888),  p.  104.)  One  great  difficulty 
in  studying  intermetallic  compounds  by  chemical 
means  is  that  even  if  definite  crystals  can  be  iso¬ 
lated  from  an  alloy  by  using  suitable  solvents  it  is 
found  that  their  compositions  vary  according  to 
the  percentage  composition  of  the  alloy  from  which 
they  were  obtained.  Apparently  identical  crystals 
isolated  from  alloys  of  different  composition, 
though  of  the  same  crystalline  form,  will  usually 
differ  on  analysis.  In  other  words,  crystals  need 
not  be  composed  solely  of  a  definite  intermetallic 
compound.  This  seems  to  have  been  the  general 
idea,  however,  in  times  past  and  every  worker  who 
succeeded  in  isolating  definite  crystals  from  an 
alloy  promptly  assigned  to  them  a  formula.  Unless 
a  compound  is  indicated  by  the  appearance  of  a 
summit  in  the  freezing-point  curve,  and  unless  the 
alloy  corresponds  in  percentage  composition  exact- 


24 


ALLOYS  AS  SOLUTIONS. 


ly  to  this  summit,  it  is  more  than  likely  that  the 
crystals  isolated,  though  isomorphous  with  those 
of  the  compound,  are  not  of  exactly  the  same  com¬ 
position.  This,  in  general,  no  doubt  accounts  for 
the  “discoveries”  from  time  to  time  of  possibly 
ten  Al-Cu  compounds  and  nearly  as  many  Al- 
Mo  compounds  of  the  types  Rx-R'y,  where  x  and 
y  may  be  any  number  from  one  to  twenty.  To  the 
same  category  belong  the  numerous  carbides  of 
iron.  I  have  discovered  six  in  the  literature,  but 
not  in  iron,  the  existence  of  all  but  one  of  which 
is  denied.  For  this  very  reason,  namely,  the  im¬ 
probable  existence  of  most  of  the  intermetallic  com¬ 
pounds  mentioned  in  technical  literature  of  the  past 
ten  years,  I  hesitate  to  give  the  formula,  W  Ah,  to 
some  beautiful  hexagonal  crystals,  which  are  so  re¬ 
fractory  that  neither  aqua  regia,  nor  aqua  regia  and 
sulphuric  acid,  nor  fused  sodium  carbonate  scarcely 
attacks  them,  and  whose  composition  agrees  very 
closely  with  that  formula. 

Cooling  Curves. — During  the  cooling  of  a  molten 
alloy,  various  constituents  may  crystallize  out  suc¬ 
cessively;  definite  compounds  which  are  stable  only 
at  high  temperatures  may  split  up  into  simple  con¬ 
stituents;  or,  new  combinations,  impossible  at  high 
temperatures,  may  be  formed  as  the  temperature 
falls.  All  such  molecular  changes  are  accompanied 
by  corresponding  thermal  effects,  such  as  the  dis¬ 
engagement  or  the  absorption  of  heat.  By  the  accu¬ 
rate  measurement  of  the  temperature  at  which  these 
changes  take  place  we  obtain  most  valuable  in¬ 
formation  in  regard  to  the  molecular  movements 
in  the  mass.  The  Le  Chatelier  pyrometer  used  in 
conjunction  with  an  auto-photographic  recording 
device  seems  best  suited  to  measure  and  record 
these  changes.  The  general  principles  of  thermo¬ 
electric  pyrometry  are  too  well  known  to  require 
discussion  here.  For  reference,  however,  I  could 
not  do  better  than  to  refer  to  H.  Le  Chatelier  and 
O.  Boudouard’s  volume,  “Mesure  des  Tempera¬ 
tures  Elevees,”  or  to  its  English  translation  with 
supplement  by  George  K.  Burgess. 


ALLOYS  AS  SOLUTIONS. 


25 


The  instruments  used  by  Sir  William  Roberts- 
Austen  in  his  private  laboratory  have  been  de¬ 
scribed  fully  in  the  Reports  of  the  Alloys  Research 
Committee  of  the  Institution  of  Mechanical  Engi¬ 
neers.  During  several  months  that  it  was  my  privi¬ 
lege  to  work  in  his  laboratory  I  made  use  of  a  record¬ 
er  designed  by  Dr.  Stansfield  and  constructed  by  the 
Cambridge  Instrument  Company.  For  a  description 
of  this,  with  illustrations,  see  the  Journal  of  the 
Franklin  Institute,  January,  1902.  Neither  of  these 
forms  of  recorder  is  perfect,  and  many  improvements 
will  be  required  to  produce  an  instrument  that  is  en¬ 
tirely  satisfactory.  Fortunately  it  is  not  the  accuracy 
of  these  instruments,  but  certain  inconveniences  of 
manipulation  that  are  at  fault.  In  using  any  thermo¬ 
couple,  with  or  without  a  recorder,  it  is  necessary 
to  calibrate  frequently.  The  temperatures  usually 
taken  as  “fixed  points”  are  the  boiling  points  of 
water  (ioo°),  naphthaline  (218°),  mercury  (356.7°), 
and  sulphur  (444.50);  the  melting  points  of  tin 
(232°),  lead  (32 90),  aluminum  (655°),  gold  (1064°), 
and  copper  (1083°).  Not  all  of  these  need  be  de¬ 
termined  for  a  single  calibration;  three  or  four 
points  if  determined  with  great  accuracy  will  suffice. 
Other  points  may  be  determined  by  way  of  verifi¬ 
cation,  and  it  is  convenient  as  well  to  have  a  large 
variety  of  materials  to  select  from  so  that  for  any 
special  work  the  curve  may  be  calibrated  by  means 
of  substances  whose  melting  or  boiling  points  lie 
near  the  temperatures  at  which  we  desire  to  oper¬ 
ate.  For  instance,  one  would  not  ordinarily  use 
the  temperatures  ioo°,  218°  and  232°  in  construct¬ 
ing  a  curve  for  use  above  iooo°,  but  would  choose 
655°,  1064°  and  1083°,  or  other  temperatures  in  this 
region  which  are  well  established  as  the  melting 
point  of  potassium  sulphate  (1084°),  or  sodium  car¬ 
bonate  (850°). 

In  using  the  thermo-couple  to  obtain  a  cooling- 
curve  of  a  metal  or  alloy  it  is  only  necessary  to 
insert  the  couple,  suitably  protected  by  a  fire-clay 
or  porcelain  tube,  into  the  molten  mass.  A  cur¬ 
rent  of  electricity  is  generated  whose  electro  mo- 


26 


ALLOYS  AS  SOLUTIONS. 


tive  force  is  approximately  proportional  to  the  tem¬ 
perature,  or  rather  the  current  gives  us  a  measure  of 
the  difference  in  temperature  between  the  hot  and 
cold  junctions  of  the  couple.  If  now  this  current 
be  passed  through  a  reflecting  galvanometer,  the 
beam  of  light  moves  rapidly  from  left  to  right 
until  it  attains  a  deflection  proportional  to  the 
temperature  measured.  This  beam  of  light  falls 
upon  the  scale  which  has  been  calibrated  by  the 
establishment  of  certain  fixed  points  as  just  ex¬ 
plained.  If  the  beam  passes  through  a  narrow 
horizontal  slit  in  a  recording  device,  behind  which 
a  photographic  plate  is  rising  or  falling  at  a  uni¬ 
form  rate,  we  obtain  a  cooling-curve  whose  co-or¬ 
dinates  are  time  and  temperature.  The  type  of 
curves  given  by  a  pure  metal,  by  surfusion  and  im¬ 
pure  metals  has  already  been  shown  in  Fig  2. 
Two  other  classes  of  alloys  give  a  cooling-curve 
identical  with  that  of  a  pure  metal,  viz. :  eutectic 
alloys  and  intermetallic  compounds,  that  is  these 
three  classes  of  substances  freeze  at  a  single  tem¬ 
perature.  The  nature  of  a  eutectic  will  be  dis¬ 
cussed  later;  suffice  it  here  to  define  it  as  that  alloy 
of  a  series  which  has  the  lowest  freezing-point, 
which  is  constant  in  composition,  and  which  is 
not  a  chemical  compound  of  the  metals  which 
compose  it.  In  Fig.  5,  the  curve  A  B  represents 
that  of  a  homogeneous  solid  solution.  It  shows 
but  one  “break, ”  though  the  temperature  does 
not  remain  constant  during  the  whole  period  of 
solidification,  as  in  the  case  of  pure  metals,  com¬ 
pounds  and  eutectics.  In  A'  B'  is  shown  a  curve 
with  two  “breaks/’  the  upper  one  resembling  that 
of  a  solid  solution  and  the  lower  one  shows  the 
form  of  eutectic.  The  first  abrupt  change  in  di¬ 
rection  in  either  of  these  curves  indicates  the  tem¬ 
perature  at  which  solidification  begins.  As  the 
excess  of  one  metal  solidifies  the  concentration  of 
the  residual  fluid  is  increased  with  respect  to  the 
metal  which  is  not  in  excess.  The  ultimate  cause 
of  the  change  in  direction  is  the  liberation  of  the 
latent  heat  of  solution  or  of  fusion. 


ALLOYS  AS  SOLUTIONS. 


27 


As  is  well  known,  when  concentrated  hydro¬ 
chloric  acid  (solution  of  hydrochloric  acid  gas  in 
water)  is  boiled,  the  hydrochloric  acid  gas  evap¬ 
orates  faster  than  the  water.  If  dilute  hydro¬ 
chloric  acid  is  boiled,  the  water  evaporates  relative¬ 
ly  faster  than  the  gas.  The  inevitable  result  is 


that  at  a  certain  concentration  hydrochloric  acid 
and  water  vapor  leave  the  liquid  at  a  rate  pro¬ 
portional  to  their  concentration;  thereafter  the 
composition  of  the  liquid  remains  constant  and 
the  boiling  point  fixed. 

A  somewhat  analogous  condition  is  that  of  the 
solidification  of  two  metals,  M  and  N.  Either  of 
them  may  be  considered  as  the  solvent.  If  M  is 
the  solvent  and  N  the  dissolved  substance,  then 
on  cooling  such  alloy,  M  crystallizes  first  re¬ 
taining  some  N  in  solid  solution.  Similarly  an 
alloy  containing  much  N  and  little  M  begins  to 
solidify  by  the  separation  of  crystals  of  nearly  pure 


28 


ALLOYS  AS  SOLUTIONS. 


N  containing  M  in  solid  solution.  As  M 
or  N  begin  to  separate  out  in  solid  form  the  re¬ 
maining  liquid  portions  become  concentrated  in 
respect  to  N  and  M  respectively.  The  freezing- 
point  of  the  remaining  liquid  is  thus  continually 
lowered.  Eventually  either  of  these  classes  of  al¬ 
loys  reaches  a  concentration  greater  than  that  of 
a  saturated  solid  solution  and  then  there  crys¬ 
tallizes  out  simultaneously  two  solid  solutions,  one 
of  which  is  a  saturated  solid  solution  of  M  in  N 
and  the  other  a  saturated  solid  solution  of  N  in 
M.  In  any  one  series  of  metals  this  takes  place 
at  a  definite  fixed  temperature  which  is  the  lowest 
freezing  point  in  the  series  and  no  matter  what 
the  original  composition  of  the  alloy,  the  part 
solidifying  at  this  constant  temperature  is  uni¬ 
form  in  composition.  This  is  the  “eutectic”  al¬ 
loy;  it  will  be  considered  more  fully  later  in  this 
paper. 

Some  students  of  alloys  consider  that  at  tem¬ 
peratures  between  the  first  separation  of  solid  and 
the  point  at  which  the  residue  solidifies  as  a  whole, 
there  does  not  exist  a  homogeneous  condition  in 
the  still  liquid  portions.  They  liken  alloys  within 
this  range  to  an  emulsion,  or  conjugate  solutions, 
etc.,  simply  because  ultimately  the  mass  solidifies 
in  a  banded  or  laminated  mass  characteristic  of 
eutectics.  It  is  not  necessary,  however,  to  con¬ 
sider  that  the  two  solid  solutions  constituting  the 
ordinary  eutectic  existed  as  such  in  an  emulsified 
state  above  the  eutectic  point.  To  be  sure,  the 
microscope  reveals  a  very  marked  separation  of 
the  alloy  into  two  constituents  (when  there  is  any 
eutectic),  usually  with  characteristically  laminated 
structure,  but  these  two  components  show  no 
tendency  to  separate  into  layers  when  the  alloy  is 
kept  for  a  long  time  just  above  its  eutectic  point; 
on  the  other  hand,  the  magnitude  of  the  eutectic 
structure  is  markedly  influenced  by  the  slowness 
of  cooling.  Furthermore  there  is  always  an  evo¬ 
lution  of  heat  during  the  freezing  of  a  eutectic 


ALLOYS  AS  SOLUTIONS. 


29 


which  is  very  decided  and  may  well  be  accounted 
for  by  such  a  marked  molecular  rearrangement  as 
the  formation  of  a  laminated  structure  from  a 
homogeneous  mass  would  involve,  and  lastly,  we 
might  cite  some  concrete  evidence  on  this  point  which 
was  brought  to  my  attention  by  Mr.  William  Camp¬ 
bell.  As  is  well  known  the  constituent  of  steel 
known  as  martensite  which  is  a  solid  solution  of 
iron  carbide  in  iron  is  capable  of  splitting  up  at 
about  690°  C.  or  nearly  8oo°  C.  below  its  freezing- 
point  into  the  beautifully  characteristic  structure 
known  as  pearlite;  accompanying  this  change  is  a 
marked  evolution  of  heat,  the  critical  point  An 
of  Osmond.  If  such  a  decided  transformation 
can  take  place  in  solid  steel,  it  surely  ought  not 
to  be  thought  improbable  that  a  similar  one  could 
take  place  in  an  alloy  at  the  instant  of  final  solidi¬ 
fication  when  the  molecular  freedom  of  the  par¬ 
ticles  is  doubtless  many  times  as  great  as  in  the 
example  cited. 

Molecular  Depression  of  the  Freezing-Point  of 
Metals. — In  1889,  Prof.  Ramsey  determined  the 
molecular  weight  of  many  of  the  metals  by  the 
method  of  measuring  the  change  in  vapor  pressure 
when  certain  known  weights  of  solid  metals  were 
dissolved  in  mercury.  He  also  made  determinations 
at  the  boiling  point  of  mercury.  From  these  ex¬ 
periments  he  states  “that  it  would  appear  legiti¬ 
mate  to  infer  that  in  solution,  as  a  rule,  the  atom 
of  a  metal  is  identical  with  its  molecule.” 

Messrs.  Heycock  and  Neville  found  that  when 
two  metals  were  melted  together,  considering  M 
as  solvent  and  N  as  the  dissolved  metal,  then 

(1)  The  freezing-point  of  M  is  lowered, — the 
most  frequent  result. 

(2)  The  freezing-point  of  M  is  raised,  e.  g.,  silver 
in  cadmium,  and  antimony  in  tin. 

(3)  The  freezing-point  is  unchanged,  e.  g.,  thal¬ 
lium  in  lead,  and,  I  believe,  within  narrow  limits, 
silver  in  gold. 

But  these  skillful  investigators,  to  whom  we  are 


30 


ALLOYS  AS  SOLUTIONS. 


so  much  indebted  for  their  careful  researches  upon 
alloys,  went  further  and  showed  that  none  of  these 
cases  contradicted  Van’t  Hoff’s  theory  of  solution. 
This  they  did  in  each  case  by  filtering  off  the  part 
first  solidifying.  In  the  first  case  the  more  fusible 
filtrate  was  found  to  be  richer  in  the  dissolved 
metal  than  were  the  first  crystals.  In  the  second 
instance  the  crystals  were  richer  in  the  dissolved 
metal;  that  is,  the  excess  of  solvent  was  not  first  to 
separate  in  the  solid  form  but  what  was  probably 
a  definite  compound  of  the  two  metals  having  a 
higher  melting  point  than  the  metal  M,  which  was 
assumed  to  be  the  solvent.  In  the  third  case  there 
was  absolutely  no  separation  of  the  two  metals 
during  cooling;  that  is,  the  first  crystals  and  the 


fluid  part  were  identical  in  composition.  That  is, 
the  two  metals  in  these  instances  torm  Isomor- 
phous  mixtures.  In  all  their  experiments,  Messrs. 
Heycock  and  Neville  were  dealing  with  dilute  so¬ 
lutions  in  which,  as  in  the  case  of  salts  dissolved  in 
water,  it  might  be  expected  that  the  molecules  of 
the  dissolved  substance  would  obey  the  laws  of 
gases.  When  their  experiments  were  conducted 
quantitatively,  Heycock  and  Neville  found  that  two 
of  the  empirical  laws  of  Coppet  and  Raoult  hold 
good  for  alloys,  viz: 


ALLOYS  AS  SOLUTIONS. 


31 


(1)  For  moderate  concentration  the  fall  of 
freezing  point  is  proportional  to  the  weight  of  dis¬ 
solved  substance  present  in  a  constant  weight  of 
solvent,  and 

i  7 

(2)  When  the  falls  produced  in  the  same  solvent 
by  different  metals  are  compared,  it  is  found  that 
a  molecular  weight  of  a  dissolved  metal  produces 

+  the  same  fall  whatever  the  metal  is. 

In  these  experiments  tin  was  the  solvent  and  it 
was  assumed  that  the  metals  are  monatomic  in 
solution  or  that  their  molecules  are  of  one  type, — 
Rn^^when  n  is  constant  and  probably  equal  to  1. 

The  third  law  is  probably  incorrect,  for  it  as¬ 


sumes  that  if  a  constant  number  of  molecules  of  sol¬ 
vent  be  employed,  the  fall  is  independent  of  the 
nature  of  the  solvent.  The  solvent  in  the  case  of 
metals  often  tends  to  chemical  combination  with 
the  other  metal.  Thus  gold  in  100  atoms  of  sodium 
gave  40  C.  depression  in  the  freezing-point;  in  tin, 
3°  C.  and  in  potassium  only  i.8°  C. 

Freezing-point  Curves . — One  must  not  confuse 
cooling-curves  and  freezing-point  curves.  The 


32  ALLOYS  AS  SOLUTIONS. 

former,  as  already  pointed  out,  result  during  the 
cooling  of  a  single  metal  or  alloy  and  time  and 
temperature  are  the  co-ordinates  of  such  curves. 
When  a  series  of  such  curves  are  obtained  for  any 
pair  of  metals  mixed  in  all  proportions  from 
o  to  ioo  per  cent  M  in  ioo  to  o  per  cent  N, 
and  the  critical  points  are  plotted  in  a  diagram  of 
which  the  co-ordinates  are  temperature  and  com¬ 
position  we  get  what  is  known  as  a  freezing-point 
curve.  That  is,  the  points  at  which  all  possible 
combinations  of  the  two  metals  freeze  are  indicated 
more  or  less  accurately.  The  more  points  in  the 
curve  actually  determined,  the  more  likely  becomes 
the  accuracy  of  the  other  points.  Subsidiary 
points  in  the  cooling-curves  may  also  be  plotted, 
though  they  are  not  strictly  speaking  freezing- 


Ax  By  Ax  By' 


points.  Figure  6  is  taken  from  a  paper  upon  “La 
Constitution  des  Alliages  Metalliques,,  by  Rob- 
erts-Austen  and  Stansfield  which  was  presented  at 
the  Physical  Congress,  Paris,  1900.  The  vertical 
co-ordinate  is  temperature  and  the  horizontal  one 
represents  both  time  (for  cooling-curves)  and 
composition  (for  the  freezing-point  curve).  It 
shows  graphically  how  a  complete  freezing-point 
curve  is  constructed  from  cooling-curves.  It  will 
be  noticed  that  the  metals  A  and  B  forming  the 
alloy  give  cooling-curves  of  the  type  of  pure  met¬ 
als.  In  the  curves  showing  a  eutectic  break,  it 


ALLOYS  AS  SOLUTIONS. 


33 


will  be  noticed  that  the  magnitude  of  the  eutectic 
break  as  compared  with  the  upper  or  freezing- 
point  break  becomes  relatively  greater  as  we  ap¬ 
proach  the  composition  of  the  alloy  c  which  is  pure 
eutectic  and  gives  a  cooling-curve  identical  in 
form  with  those  of  A  and  B.  In  fact  this  ratio 
gives  a  rough  approximation  of  the  proportion  of 
the  alloy  which  consists  of  solid  solution  separating 
at  the  first  break  and  eutectic  alloy  crystallizing 
at  the  fixed  temperature  indicated  by  the  horizon¬ 
tal  part  of  the  cooling-curve;  or,  in  other  words, 
if  the  alloy  consists  mostly  of  solid  solution  and  a 
little  eutectic,  it  will  take  the  former  longer  to 
solidify  than  it  does  the  latter  and  the  cooling- 
curve  indicates  this  roughly.  Sometimes  more 
than  two  breaks  occur  in  a  cooling-curve,  indicat¬ 
ing  changes,  allotropic  or  otherwise,  occurring  in 
the  solid. 

Freezing-point  curves  conform  in  general  to  three 
types,  as  shown  in  Fig.  7.  These  curves  are  not 
drawn  exactly  to  scale,  but  are  approximately  cor¬ 
rect.  Gold  and  silver  alloys  form  perfectly  isomor- 
phous  mixtures,  and  their  freezing  points  give  almost 
a  straight  line  joining  the  freezing  points  of  the  pure 
metals.  Copper  and  silver  give  the  typical  curve  of 
two  metals,  which  mix  in  all  proportions,  but  do  not 
unite  chemically.  Antimony  and  copper  unite  to  give 
the  highly  colored  compound  known  as  regulus  of 
Venus,  Cu2  Sb.  *This  compound  is  indicated  in  the 
cooling-curve  by  the  intermediate  summit.  In  gen¬ 
eral  such  a  summit  will  be  found  to  occur  at  a  for¬ 
mula  percentage,  and  indicates  an  intermetallic  com¬ 
pound.  To  explain  such  a  curve,  we  have  only  to 
consider  it  as  made  up  of  separate  sections,  as 
indicated  in  the  figure  by  a  dotted  line.  The  inter¬ 
metallic  compound  has  a  melting  point  of  its  own, 
quite  independent  of  that  of  the  constituent  metals. 
It  may  be  higher,  lower  or  intermediate  as  com- 


*The  formula  usually  given  in  the  past  was  Cu5Sb2,  but 
Stead  says  the  purple  constituent  is  Cu2Sb. 


lioo  a 


34  ALLOYS  AS  SOLUTIONS. 


100/c 


ALLOYS  AS  SOLUTIONS. 


35 


pared  with  the  constituent  metals  which  compose  it'. 
In  such  a  curve  as  the  Sb-Cu  one,  we  may  consider 
that  one  series  of  alloys  is  composed  of  mixtures  of 
Sb  and  Sb  Cu2,  and  the  other  of  Sb  Cu2  and  Cu. 
We  are  virtually  dealing  with  two  distinct  series  of 
alloys,  each  of  which  taken  separately  is  of  the  simple 
type  illustrated  by  the  Ag-Cu  curve.  In  the  Sb-Sb- 
Cu2  portion  of  the  curve  there  is  no  free  copper,  and 
in  Cu-Sb  Cu2  portion  of  the  curve  there  is  no  free 
antimony.  Dissociation  of  the  compound  might  make 
some  modification  of  the  above  statement  necessary, 
and,  indeed,  it  has  been  suggested  that  dissociation 
causes  a  summit  to  appear  rounded  instead  of  an¬ 
gular.  Curves  containing  more  than  one  summit 
may  be  similarly  resolved  into  series  of  two  com¬ 
ponents,  which  may  be  respectively  a  metal  and  an 
intermetallic  compound,  or,  if  between  two  summits, 
the  components  are  both  intermetallic  compounds. 
An  ideal  representation  of  such  a  curve  is  shown  in 
Fig.  8.  In  this  figure  we  see  that  alloys  whose  com¬ 
positions  fall  between  i  and  2  consist  of  metal  A  and 
compound  Ax  By ;  alloys  between  2  and  3  consist  of 
components  Ax  By,  and  Ax'  By' — they  contain  no 
free  A  or  B ;  alloys  between  3  and  4  consist  of  com¬ 
pound  Ax'  By'  and  metal  B.  Each  of  these  pairs  of 
components  has  its  melting  point  depressed  by  the 
presence  in  it  of  the  adjacent  component.  Our  Cu- 
A1  curve  (Fig.  9)  may  be  explained  in  this  way: 
Two  summits  occur  at  the  compositions  correspond¬ 
ing  to  Cu2  Ah  and  A1  Cu3  (48.49%  Cu,  and  87.6%  Cu 
respectively).  Le  Chatelier  thinks  he  has  detected 
with  the  microscope  at  least  four  compounds.  Our 
curve  gives  no  indication  of  them.  Our  own 
microscopic  study  of  these  alloys  is  not  yet  com¬ 
pleted.  Our  Al-Sb  curve  (Fig.  9)  shows  the  pres¬ 
ence  of  a  compound  whose  melting  point  is  more  than 
400°  C.  above  that  of  either  constituent.  Its  formula 
is  Sb  A1  (81.6%  Sb).  In  this  series  of  alloys  we  are 
dealing  with  two  pairs  of  constituents,  Al-Al  Sb  and 
A1  Sb-Sb.  The  compound  seems  to  be  almost  insolu¬ 
ble  in  either  of  the  pure  metals,  but  on  theoretical 


36  #  ALLOYS  AS  SOLUTIONS. 

grounds  we  should  expect  to  find  a  slight  depression 
in  the  freezing  point  of  each  metal  as  indicated  in 
the  curve.  These  points  have  not  as  yet  been  experi¬ 
mentally  detected.  The  tin-aluminum  curve  shows  a 
fall  of  30  or  40  C.  by  the  addition  of  0.5%  Al,  while 
with  increased  additions  of  aluminum  the  freezing 
point  is  raised  300°  C.  at  a  concentration  of  10%  Al, 
90%  Sn. 


FIG.  10. 

If  in  a  certain  series  of  alloys  we  get  a  cooling 
curve  with  a  summit  occurring  at  a  formula  percent¬ 
age,  and  examine  the  alloys  whose  freezing  points 
give  rise  to  that  summit,  we  shall  find  that  in  general 
an  intermetallic  compound  in  the  pure  state  presents 
under  the  microscopic  a  homogeneous  mass  made  up 
of  crystals,  all  of  which  are  the  same.  As  we  de- 


ALLOYS  AS  SOLUTIONS.  37 

scend  from  the  summit  on  either  branch  of  the  curve 
we  find  these  crystals  becoming  less  and  less  in  num¬ 
ber  and  size,  and  at  the  next  angle  in  the  curve  they 
disappear  entirely.  This  is  illustrated  by  the  accom¬ 
panying  photographs  of  antimony-aluminum  alloys. 
These  microphotographs  were  made  by  Mr.  Camp¬ 
bell. 

Fig.  io. — 20  per  cent  Sb,  80  per  cent  A1  X  33  diam¬ 
eters,  oblique  illumination,  shows  crystals  of  Sb  A1 
in  granular  ground  mass  of  nearly  pure  Al. 


FIG.  ii. 

Fig.  ii. — 50  per  cent  Sb,  50  per  cent  Al  X  33  diam¬ 
eters,  oblique.  Crystals  of  Sb  Al  increasing  in  size 
and  amount,  ground  mass  decreasing. 

Fig.  12. — 82  per  cent  Sb,  18  per  cent  Al  X  16  diam- 


ALLOYS  AS  SOLUTIONS. 


38 

eters,  verticle  illumination.  This  photograph  shows 
nearly  pure  Sb  Al. 

Fig.  13. — 85  per  cent  Sb,  15  per  cent  Al  X  33  diam¬ 
eters,  vertical  illumination.  We  have  passed  the  sum¬ 
mit  of  the  curve;  the  ground  mass  of  aluminum  has 
disappeared  and  some  free  antimony  is  seen. 

Fig.  14. — 95  per  cent  Sb,  5  per  cent  Al  X  33  diam¬ 
eters,  oblique.  Sb  Al  crystals  diminishing  in  quan¬ 
tity,  and  ground  mass  of  Sb — probably  containing  a 
little  Sb  Al  in  solid  solution — is  conspicuous. 

Regarding  this  microscopic  evidence,  supporting 


FIG.  1 2. 

the  pyrometric  evidence,  Mr.  Neville  says:  ‘These 
criteria  taken  together,  (1)  the  occurrence  of  a  sum¬ 
mit  at  a  formula  percentage,  (2)  the  presence  of  large 


ALLOYS  AS  SOLUTIONS.  39 

crystals  of  the  same  kind,  decreasing  in  amount  as 
we  descend  the  branch  on  either  side,  are  an  absolute 
proof  of  the  reality  of  a  compound.” 

The  Nature  of  Eutectics. — In  several  portions  of 
this  paper  eutectic  alloys  have  been  mentioned,  a 
term  for  which  we  are  indebted  to  Dr.  Guthrie,  who 
used  it  to  designate  the  most  fusible  alloy  of  a  series 
— the  one  which  freezes  last.  There  are  certain  points 
in  which  eutectics  may  differ,  and  upon  these  differ¬ 
ences  they  may  admit  of  classification.  Every  eutec¬ 
tic,  however,  possesses  the  following  properties : 

1.  It  is  of  uniform  composition  in  any  one  series 
of  alloys. 

2.  Its  freezing  point  is  constant  throughout  any  one 
series. 

3.  The  freezing  point  is  the  lowest  in  the  series. 

4.  It  is  not  a  chemical  compound. 

The  composition  of  a  eutectic  may  and  occasion¬ 
ally  does  correspond  to  some  simple  atomic  ratio. 
This  coincidence,  though  striking,  does  not  prove  the 
presence  of  a  compound  as  the  sole  constituent  of  a 
eutectic.  Usually  it  possesses  a  laminated  micro¬ 
structure,  and  often  requires  very  high  magnifica¬ 
tion  to  detect  the  two  constituents.  Mr.  Stead,  in  his 
splendid  paper  upon  iron  and  phosphorus,  gives  his 
ideas  upon  the  subject  of  eutectics,  and  in  addition 
to  what  has  just  been  said  as  to  the  essentials  of  a 
eutectic,  he  adds  some  remarks  upon  what  a  eutec¬ 
tic  may  be:  (i)  “It  may  consist  of  two  or  more 
metals  which  do  not  unite  chemically,  or  (2)  of  a 
metal  and  a  definite  compound  (containing  that 
metal),  or  (3)  possibly  of  two  or  more  definite  com¬ 
pounds.  (4)  It  may  consist  of  a  mixture  of  a  solid 
solution  of  one  metal  in  another  and  a  free  metal. 
(5)  It  may  contain  a  solid  solution  of  a  definite 
metallo-metallic  salt  (intermetallic  compound)  and 
that  same  metallo-metallic  salt  in  the  free  state.  (6) 
It  may  possibly  consist  of  two  solid  solutions.”  The 
last  of  these  statements,  which  he  qualified  by  the 
word  “possibly,”  seems  to  me  to  be  the  most  impor¬ 
tant  of  all ;  and  since  I  am  unwilling  to  admit  that  any 


40 


ALLOYS  AS  SOLUTIONS. 


metal  or  intermetallic  compound  ever  separates  ab¬ 
solutely  pure,  those  conditions  in  which  Mr.  Stead 
speaks  of  the  separation  of  pure  substances  seem  to 
me  to  be  inaccurate.  The  form  of  the  freezing  point 
curves  and  the  explanation  of  them  according  to 
Prof.  Roozeboom  renders  it  highly  improbable  that 
any  strictly  pure  material  separates  at  the  freezing 


FIG.  13. 


point  of  the  eutectic.  The  most  that  we  can  say  of 
the  constituents  of  a  eutectic  is  that  in  certain  cases 
they  are  solutions  of  extreme  dilution.  In  cases 
where  the  freezing  point  curve  lies  almost  entirely 
above  the  melting  point  of  either  or  both  constituents, 
but  in  which  there  is  a  slight  depression  of  the  freez¬ 
ing  point  by  very  small  additions  of  one  metal  to  the 


ALLOYS  AS  SOLUTIONS. 


41 


other,  as  in  the  case  of  tin-aluminum,  previously 
mentioned,  and  when  this  depression  is  further  indi¬ 
cated  in  a  well-marked  eutectic  break  in  other  alloys 
of  the  series,  it  may  be  possible  that  the  eutectic  is  a 
single  solid  solution.  The  quantity  of  dissolved  metal 
in  such  a  eutectic  might  not  be  enough  to  change  the 
type  of  cooling  curve  seriously,  and  probably  exceed¬ 
ingly  high  magnification  would  fail  to  resolve  such 
an  eutectic  into  two  components,  for  it  crystallizes 
isomorphously  with  the  pure  tin,  hence  it  seems  that 
no  two  juxtaposited  constituents  are  there.  This  con¬ 
ception  of  a  eutectic  is  not  at  variance  with  the  four 
essentials  already  enumerated.  While  not  speaking 


FIG.  14. 


positively  on  this  point,  I  think  it  reasonable,  and  am 
not  sure  but  that  in  such  a  curve  as  the  Sb-Sn  curve, 
where  all  the  points  seem  to  lie  above  the  melting 
point  of  tin,  we  must  consider  pure  tin  as  the  eutectic 
— it  melts  lowest  for  the  series,  it  is  of  constant  com¬ 
position,  and  it  is  not  a  chemical  compound. 

We  have  now  indicated  many  points  of  resemblance 
between  metallic  mixtures  and  ordinary  solutions. 
One  metal  diffuses  into  another  like  a  salt  into  water ; 
like  two  liquids,  they  may  be  perfectly  miscible  or 
form  layers;  the  layers  are  not  pure,  but  each  con¬ 
tains  a  little  of  the  other  in  solution;  in  general,  the 
solubility  increases  with  the  temperatures.  They  will 


42 


ALLOYS  AS  SOLUTIONS. 


flow  under  pressure ;  they  may  or  they  may  not  react 
chemically  when  brought  into  intimate  association  by 
fusion  or  pressure;  the  molecular  mobility  increases 
with  the  temperature;  upon  cooling  of  binary  alloys 
we  observe  phenomena  strongly  suggestive  of  the 
freezing  of  salt  solutions ;  the  depression  of  the  freez¬ 
ing  point  of  one  metal  when  another  is  added  follows 
the  laws  of  Coppet  and  Raoult,  and  the  eutectic  re¬ 
minds  us  very  much  of  the  “cryohydrates”  of  ordi¬ 
nary  solutions.  Scarcely  another  point  of  resem¬ 
blance  is  needed  but  one  is  at  hand,  and  a  very  im¬ 
portant  one,  viz.,  the  phase  rule  applies  quite  as  well 
to  the  explanation  of  conditions  of  equilibrium  in 
alloys  as  it  does  to  the  explanation  of  similar  problems 
in  regard  to  liquid  solutions.  The  classification  of 
the  systems,  however,  requires  some  modification  be¬ 
fore  the  generalizations  of  Trevor  apply,  and  it  is 
practically  somewhat  difficult  to  ensure  complete 
equilibrium  in  an  alloy — the  cooling  is  usually  much 


too  rapid  to  allow  of  the  establishment  of  equilibrium 
in  the  solid  mass.  If  equilibrium  has  been  estab¬ 
lished,  then  the  number  of  distinct  substances  in  the 
mass  will  depend  upon  the  number  of  constituents 
which  enter  into  the  composition  of  the  mass.  In  my 
paper  already  referred  to  in  the  Journal  of  the  Frank¬ 
lin  Institute,  February,  1902,  I  have  shown  how  the 


ALLOYS  AS  SOLUTIONS. 


43 


adaptation  of  the  phase  rule  to  alloys  is  accomplished, 
and  as  this  is  more  a  chemical  conception  than  one  of 
direct  interest  to  engineers,  it  need  not  be  dwelt 
upon  here.  The  phase  rule  does  not  tell  us  the  num¬ 
ber  of  phases  in  which  one  or  two  components  may 
exist,  but  how  many  of  them  may  exist  simultaneous¬ 
ly  in  equilibrium.  Thus  pure  iron  may  exist  in  three 
solid,  and  one  liquid  phase,  but  never  in  all  four  at 
one  time.  Again  the  iron-carbon  alloys,  according  to 
Roozeboom,  may  exhibit  at  least  seven  phases,  viz., 
carbon,  alpha-,  beta-and  gamma-iron,  liquid  solution, 
solid  solution  of  carbon  in  gamma-iron,  and  cementite 
or  iron-carbide.  The  provisions  of  the  phase  rule, 
however,  tell  us  that  only  three  of  these  may  exist  in 
equilibrium  at  any  given  temperature  and  concentra¬ 
tion.  Professor  Roozeboom’s  explanation  of  a  freez¬ 
ing-point  curve  considered  as  an  equilibrium  curve  is 
of  great  interest  in  helping  to  make  clear  the  series 
of  phenomena  which  are  exhibited  during  the  actual 
solidification  of  molten  alloys.  His  original  explana¬ 
tion  was  given  in  connection  with  the  highly  compli¬ 
cated  iron-carbon  curves  of  Roberts- Austen.  It  has 
seemed  to  me  that  a  simpler  figure  would  lend  itself 
more  readily  to  this  explanation  and  the  accompany¬ 
ing  figure  shows  the  type  of  curves  which  gold-copper 
and  copper-silver  alloys  give,  i.  e.,  pairs  of  metals 
soluble  in  each  other  in  all  proportions  but  not  form¬ 
ing  any  compounds.  In  Fig.  15  the  curve  a  e  c  repre¬ 
sents  the  temperatures  at  which  for  each  concentra¬ 
tion  solidification  begins.  The  curve  a  b  d  c  shows 
the  temperature  at  which  solidification  is  complete. 
Except  for  the  straight  portion  of  this  curve,  b  d,  the 
exact  position  of  it  is  a  matter  of  conjecture.  Accord¬ 
ing  to  Stansfield,  its  position  may  be  calculated  with 
some  degree  of  accuracy  upon  theoretical  grounds,  if 
the  latent  heat  of  fusion  of  the  solvent  is  known  and 
assuming  that  the  dissolved  substance  is  monatomic 
in  the  liquid  state.  The  upper  line,  a  e  c  Roozeboom 
calls  the  “liquid”  curve,  and  the  lower  line,  a  b  e  d  c 
he  calls  the  “solid”  curve.  When  in  a  series  of  alloys 


44 


ALLOYS  AS  SOLUTIONS. 


the  liquid  and  solid  curves  have  a  maximum  or 
minimum  point  formed  by  the  branches  of  the  curve, 
they  touch  at  this  point  as  in  the  curve  before  us. 
The  point  in  our  diagram  where  the  liquid  and  solid 
curves  meet  is  the  eutectic  point.  It  means  that  the 
liquid  represented  by  the  composition  corresponding 
to  that  point  solidifies  as  a  whole  at  a  single  tempera- 
ature.  Curves  similar  to  this  may  be  used  to  explain 
such  complicated  freezing-point  curves  as  that  of  the 
Al-Cu  series,  remembering  that  the  terminii  of  the 
liquid  and  solid  curves  are  not  necessarily  pure  met¬ 
als,  but  may  be  a  metal  and  a  compound,  or  perhaps 
two  compounds.  The  more  or  less  triangular  areas, 
a  b  e  and  c  d  e  represent  mixtures  of  liquid  and  solid 
phases.  The  areas  a  b  r  f  and  c  d  g  s  are  solid  solu¬ 
tions,  and  in  the  area  b  d  f  g  we  have  a  conglomerate 
of  eutectic  alloy,  with  varying  amounts  of  crystals  of 
solid  solution  depending  upon  the  composition  of  the 
alloy  from  which  they  were  derived.  At  the  single 
composition  e,  however,  we  have  an  exception  to  this 
statement,  for  this  alloy  consists  of  eutectic  only, 
while  to  the  left  of  this  the  conglomerate  consists  of 
eutectic  plus  solid  solutions  of  N  in  M,  and  in  alloys 
represented  by  compositions  to  the  right  of  e  we 
have  eutectic  (always  constant  in  composition)  plus 
more  or  less  of  a  solid  solution  of  M  in  N. 

Above  the  liquid  curve  we  have  only  liquid  phases ; 
below  the  solid  curve  we  have  only  solid  phases,  while 
intermediate  areas  correspond  to  a  mixture  of  liquid 
and  solid  phases. 

Any  alloy  whose  composition  falls  within  the  areas 
marked  solid  solution  gives  but  one  break  in  the 
cooling  curve.  Alloys  represented  by  compositions 
falling  within  the  area  of  the  eutectic  (except  alloy 
e)  give  two  breaks  in  the  cooling-curves.  The  eutec¬ 
tic  itself  consists  of  two  solid  solutions,  whose  com¬ 
positions  are  indicated  by  compositions  correspond¬ 
ing  to  the  extreme  ends  of  the  eutectic  line,  viz., 
b  and  d.  However,  since  the  solubility  of  one  metal 
in  another  is  a  function  of  the  temperature  below 


ALLOYS  AS  SOLUTIONS.  45 

its  freezing  point  as  well  as  above,  we  may  assume 
that  the  solubility  in  the  two  components  of  the  eu¬ 
tectic  decreases  slightly  even  after  solidification,  and 
therefore  the  lines  b  f  and  d  g  are  represented  as 
slanting  rather  than  perpendicular  to  the  composition 
line.  These  lines  indicate  the  maximum  solubility  of 
either  metal  in  the  other  at  temperatures  below  the 
freezing  point  of  the  eutectic. 

Let  us  consider  the  phenomena  of  solidification  of 
an  alloy  represented  by  a  composition  on  a  e,  as  ex¬ 
plained  by  Roozeboom.  Any  solution  represented  by 
the  composition  m  begins  to  solidify  at  that  tempera¬ 
ture  by  the  separation  of  crystals  of  the  composition 
n.  They  therefore  contain  less  of  N  than  the  liquid 
alloy  m.  The  residual  alloy  has  therefore  become  en¬ 
riched  in  N,  and  its  freezing  point  is  lowered ;  its 
composition  passes  thus  from  m  to  p,  at  which  point 
solidification  is  complete,  for  at  this  point  the  tem¬ 
perature  has  fallen  till  it  encounters  the  solid  curve 
at  the  point  o.  The  point  o,  moreover,  indicates  the 
composition  of  the  last  crystals  to  solidify.  That  is, 
while  the  liquid  solution  is  changing  from  m  to  p ,  the 
composition  of  the  crystals  has  changed  from  n  to  o. 
Complete  solidification  has  thus  taken  place  through 
an  interval,  m  o.  Now  there  is  an  alloy  whose  final 
solidifying  point  is  b ;  and  according  to  Roozeboom’s 
explanation  it  is  an  alloy  of  composition,  e,  which  is 
freezing.  At  the  same  temperature,  however,  occurs 
the  solidification  of  the  solid  solution  of  composition 
d.  Finally,  then,  all  alloys  represented  by  composi¬ 
tions  between  i  and  2,  after  separating  out  crystals  of 
solid  solutions  of  N  in  M,  or  M  in  N,  represented  by 
compositions  along  the  lines  3,  1  and  2,  4,  respectively, 
become  concentrated  to  the  composition  e,  where  sat¬ 
urated  solid  solutions  of  compositions  b  and  d  solid¬ 
ify  side  by  side,  constituting  the  eutectic.  That  is, 
an  alloy  the  vertical  projection  of  whose  composition 
upon  the  solid  curve  does  not  cut  the  eutectic  line, 
consists  of  a  single  solid  solution.  An  alloy  whose 
composition  projected  vertically  intersects  the  eutec¬ 
tic  line,  consists  of  a  conglomerate  of  eutectic  of  con- 


ALLOYS  AS  SOLUTIONS. 


46 

stant  composition  e  and  a  solid  solution,  whose  com¬ 
position  is  represented  by  the  horizontal  projection  of 
its  composition  upon  the  corresponding  solid  curve. 
At  temperatures  and  concentrations  lying  within  the 
areas  enclosed  by  the  liquid  and  solid  curves  there 
exists  in  equilibrium  solid  and  liquid  solutions,  e.  g., 
at  a  temperature  and  concentration  represented  on  the 
line  m  n  there  exists  simultaneously  liquid  solution 
m  and  solid  solution  n. 

Great  as  has  been  the  progress  in  alloys  research 
within  the  past  few  years,  yet  much  remains  to  be 
done.  When  many  sets  of  complete  freezing-point 
curves  have  been  determined,  when  all  the  alloys  of 
these  series  have  been  examined  metallographically, 
and  when  their  ordinary  physical  tests,  tensile  strength, 
electrical  and  heat  conductivity,  specific  gravities,  etc., 
have  been  accurately  worked  out,  then  we  may  be  able 
to  generalize  and  to  predict  properties  of  new  pairs  of 
metals  as  well  as  to  set  out  intelligently  to 
produce  new  and  useful  alloys.  It  must  be  admitted 
that  the  production  of  alloys  in  the  past  has  been 
by  the  very  wasteful  method  of  chance.  I  hope  to  see 
the  manufacture  of  alloys  become  an  exact  science  and 
that  ultimately  even  ternary  or  more  complex  alloys 
may  be  brought  within  the  range  of  applicability  of 
“natural  laws,”  which  as  yet  remain  undiscovered. 
We  are  making  progress  in  the  right  direction,  but 
American  scientists  have  not  yet  assumed  their  due 
share  of  this  task.  It  is  time  that  we  followed  the 
example  of  the  British  association,  the  Societe  d’En- 
couragement  pour  l’lndustrie  Nationale,  the  Institu¬ 
tion  of  Mechanical  Engineers  and  the  National  Physi¬ 
cal  Laboratory  of  Great  Britain,  in  promoting  the 
scientific  study  of  problems  in  connection  with  alloys. 


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